Field of the Invention
The present invention relates to a condenser microphone unit, and especially relates to a primary sound pressure-gradient type condenser microphone unit which drives a diaphragm by difference in sound pressure applied to front and rear acoustic terminals.
Description of the Related Art
A primary sound pressure-gradient type condenser microphone including acoustic terminals in front and rear of a diaphragm drives the diaphragm by difference in sound pressure applied to the front and rear acoustic terminals. A force driving a diaphragm (hereinafter called a drive force) and an upper limit of a frequency which can obtain a sufficient drive force can be determined by a distance between acoustic terminals. Specifically, in the case where a distance between acoustic terminals is short, the drive force of a diaphragm is lowered although the diaphragm operates at a high frequency. On the other hand, it is the other way around if the distance between acoustic terminals is long.
As described above, a conventional primary sound pressure-gradient type microphone has mutually opposite characteristics regarding a drive force of a diaphragm and sound collection in a high frequency band. However, these characteristics are determined when a microphone unit is designed. Therefore, it has been difficult to adjust the characteristics after the unit has been formed.
To solve the above issue, a distance between the front and rear acoustic terminals needs to be changed on the outside of the primary sound pressure-gradient type microphone unit. Specifically, the distance between the front and rear acoustic terminals can be changed if a sound wave path such as an acoustic tube is provided between the front and back acoustic terminals, and the length of the sound wave path can be changed. As a result, a drive force of a diaphragm and sound collection characteristics in a high frequency band can be easily adjusted.
Also, a primary sound pressure-gradient type microphone is conventionally used as a directional microphone. Therefore, the primary sound pressure-gradient type microphone can preferably adjust directivity.
An applicant of the present invention proposes, in JP 3975007 B1, a structure in which a baffle 52 is attached around a diaphragm 51 of a unidirectional condenser microphone unit 50 as illustrated in FIG. 5, those are stored in a recess 55a formed to an outer frame 55 of a display panel for a computer, and sound wave goes around into the recess 55a from a space (side terminal) 54 outside of the baffle 52 is collected on a rear acoustic terminal 56 side.
However, in the structure disclosed in JP 3975007 B1, a distance between front and rear acoustic terminals is changed by a size (area) of a baffle, and therefore a diameter of the recess 55a on the outer frame of a panel (opening diameter) needs to be changed, and it significantly affects a design aspect.
For example, a structure as illustrated in FIG. 6 is considered to change the distance between front and rear acoustic terminals without changing a diameter of an opening for storing a unit. An acoustic tube 61 extending forward is provided on a front surface side of a microphone unit 60, and the distance is adjusted by the length of the acoustic tube 61. In the case of FIG. 6, an opening 70a is provided on a wall 70, and the microphone unit 60 is stored from the opening 70a. 
However, in the case of such a structure, a path as illustrated in FIG. 6 needs to be secured. Specifically, a bottomed acoustic tube 63 is provided and the unit and the acoustic tube 61 are arranged therein. Also, a path through which sound wave goes into the unit rear side from the outside of a baffle plate 62 needs to be secured. Therefore, since a path of sound wave is doubly provided to the unit rear side, a wide space is needed on the rear side of the wall 70, and a structure becomes complicated.